1. Field of the Invention
The present invention relates to a heat-resistant expansive member in the form of, for example, a sheet preferable as a holding member of a ceramic honeycomb monolithic catalyst forming a catalyst converter in a low pollution engine capable of purifying the emission by oxidizing or reducing harmful components discharged from an automotive engine such as carbon monoxide, hydrocarbon and nitrogen oxides.
2. Description of the Prior Art
The ceramic honeycomb monolithic catalyst excellent in high temperature characteristic is considered to be preferred as the catalyst for achieving a low pollution engine by purifying the emission from the engine by oxidizing or reducing the harmful components discharged from the automotive engine, such as carbon monoxide, hydrocarbon and nitrogen oxides.
Since the ceramics are brittle and inferior in toughness, the ceramic catalyst as installed, is surrounded by a cushioning holding member, in a metallic casing in order to prevent the ceramic catalyst from being damaged due to a mechanical shock such as vibration occurring when the vehicle is running.
The ceramic honeycomb monolithic catalyst is exposed to the high temperature emission of the engine, and therefore a holding member is required to have an excellent heat resistance so as not to be lowered in high temperature strength. Further, since the emission is gradually heightened in temperature as the engine runs continuously, the holding member is thermally expanded depending on the temperature increase. Even in such circumstances, it is required that the holding force and cushioning properties for the ceramic honeycomb monolithic catalyst are not lowered.
As the holding member of the monolithic catalyst capable of satisfying such requirements, there is known, for example, the heat-resistant expansive sheet disclosed in the Japanese Patent Publication 61-35143.
This heat-resistant expansive sheet is composed of 40 to 65 wt % of treated vermiculite as obtained by treating particulate vermiculite with an aqueous solution of ammonium dihydrogen phosphate, 25 to 50 wt % of inorganic fibers, and 5 to 15 wt % of a binder selected from the group consisting of inorganic binders.
Table 2 and FIG. 3, show the results of tests conducted on this heat-resistant expansive sheet. As apparent from Table 2 and FIG. 3, the heat-resistant expansive sheet above-mentioned presents a relatively great negative expansion due to a creep phenomenon around 200.degree. C. to 300.degree. C. (more specifically, 200.degree. C. to 325.degree. C.) corresponding to the low temperature zone. Further, the thermal expansion amount at a temperature of 350.degree. C. to 400.degree. C. corresponding to the intermediate temperature zone, is considerably small. This causes the monolithic catalyst to be loosened, so that the holding force for the ceramic honeycomb monolithic catalyst is extremely lowered.
Also apparent from Table 2 and FIG. 3, this heat-resistant expansive sheet is restrained in thermal expansion at a temperature of 600.degree. C. or more corresponding to the high-temperature zone, thereby to lowering the holding force for the ceramic honeycomb monolithic catalyst. Thus, in the conventional heat-resistant expansive sheet, a high holding force is not to be expected in both low- and high-temperature zones, causing the monolithic catalyst to be readily loosened.
Further, since this conventional heat-resistant expansive sheet is retained in shape with an inorganic binder only, that part of the holding member exposed to the a high temperature exhaust gas flowing at high speed, may gradually fall. Accordingly, the monolithic catalyst holding performance dissapears with the passage of time. That is, this conventional sheet presents the problem that the gas-attack resisting properties are very poor.
There is also known a heat-resistant expansive sheet obtained by treating the particulate vermiculite mentioned earlier with sodium dihydrogen phosphate. In this sheet, the negative expansion in the low-temperature zone may be restrained, but the thermal expansion at the intermediate- and high-temperature zones is small. Thus, a sufficient holding force is not be expected (See Table 2 and FIG. 3).